Field of the Invention
The present invention relates to a liquid ejection head configured to eject a liquid and a liquid ejection head substrate for the liquid ejection head.
Description of the Related Art
An exemplary liquid ejection head, which is configured to eject a liquid such as ink, includes an ejection opening defining member and a liquid ejection head substrate. The ejection opening defining member has ejection openings. The liquid ejection head substrate includes a heating resistor configured to generate thermal energy for forming a bubble in the liquid. The liquid ejection head substrate has a contact portion (hereinafter, may be referred to as a “heat application portion”), which is in contact with the liquid, at a position corresponding to the heating resistor. The heating resistor heats the liquid at the heat application portion rapidly when activated. Thus, a bubble is formed in the liquid at the heat application portion. A pressure applied by the bubble causes the liquid to be ejected through the ejection opening for printing on a medium.
During the above-described process, the heat application portion of the liquid ejection head substrate may be subjected to both physical action such as an impact caused by cavitation due to bubble formation or bubble shrinkage in the liquid and chemical action caused by a liquid such as ink. A protective film covers the heating resistor so as to protect the heating resistor from the influence of such action.
When the liquid is heated at high temperature at the heat application portion, which is the contact portion where the liquid is in contact with the protective film, an additive such as a coloring material included in the liquid is decomposed and a substance having low solubility is produced. The substance having low solubility is likely to be physically adsorbed by the protective film. The physically adsorbed substance is referred to as kogation. The kogation on the protective film causes uneven heat transfer from the heating resistor to the liquid, leading to unstable bubble formation. This may result in unstable liquid ejection.
Japanese Patent Laid-Open No. 2008-105364 describes a method of cleaning the liquid ejection head. In the method, an electrode is disposed and a voltage is applied such that the protective film becomes a positive side and the electrode becomes a negative side. This causes an electrical chemical reaction between the liquid and a component of the protective film, causing the surface of the protective film to be eluted in the liquid. Thus, the kogation is eliminated.
In the elimination of the kogation by using the electrical chemical reaction, the component of the protective film is eluted rapidly in an area of the protective film adjacent to the electrode and is eluted slowly in an area of the protective film remote from the electrode. The influence of the difference in the elution speed in the protective film, which varies depending on the distance from the electrode, is reduced by separating the protective film from the electrode by a sufficient distance. However, if the distance between the protective film and the electrode is short, the difference in the elution speed is large. In this case, the thickness of the protective film may vary if cleaning of the liquid ejection head continues. The variation in the thickness of the protective film may lead to uneven heat transfer to the liquid, and the liquid ejection head may fail to stably form a bubble in the liquid. Thus, stable liquid ejection is unlikely to be maintained.
If the electrode is disposed at a position sufficiently remote from the protective film, the size of the liquid ejection head may be increased depending on the position of the electrode.
The present invention provides a liquid ejection head substrate having a small size and enabling stable liquid ejection. The stable liquid ejection is achieved by separating a protective film from an electrode by a sufficient distance to reduce variation in the elution amount, which is the amount of a component eluted from the protective film, depending on the position in the protective film.